The semiconductor integrated circuit (IC) industry has experienced rapid growth in the past several decades. Technological advances in semiconductor materials and design have produced increasingly smaller and more complex circuits. These material and design advances have been made possible as the technologies related to processing and manufacturing have also undergone technical advances. As a size of the smallest component has decreased, numerous challenges have risen. High resolution lithography processes are often one of the more important areas to decreasing feature size, and improvements in this area are generally desired. One lithography technique is extreme ultraviolet (EUV) lithography. Other techniques include X-Ray lithography, ion beam projection lithography, electron beam projection lithography, and multiple electron beam maskless lithography.
EUV lithography is a promising patterning technology for very small semiconductor technology nodes, such as 14-nm, and beyond. EUV lithography is very similar to optical lithography in that it uses a mask to print wafers. However, unlike optical lithography, EUV employs light in the EUV region, e.g., at about 13.5 nm. At the wavelength of 13.5 nm, most materials are highly absorbing. Thus, reflective optics, rather than refractive optics, are used in EUV lithography. Although existing methods of EUV lithography have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects. For example, the EUV light produced by tin plasma, such as DPP (discharge-produced plasma) and LPP (laser-produced plasma), emits some out of band (OOB) radiation. An OOB radiation is a radiation with an wavelength which is out of a pre-determined wavelength band centered on a pre-determined wavelength, such as a 2% band centered on 13.5 nm. A portion of the OOB radiation (sometimes referred to as a flare) can also arrive at the target substrate (e.g., a wafer) and cause image contrast loss. So it is desired to have further improvements in this area.